It is proposed to examine the active-site peptide of alpha-methylglucosidase from yeast, employing methyl alpha-D-glucoside in order to (a) identify the glycosylated amino acid and the neighboring amino acids. The sterochemistry of the glucoside bond will be determined to test the hypothesis that the mechanism involves a double inversion. The total amino acid sequence of the protein will be undertaken in order to localize the peptide. These studies will be extended to an associated enzyme which is produced by the same strain of yeast, which we have obtained in pure form and have shown to be very similar in kinetic and physical properties. Other glycosidases amenable to such labeling will also be studied.